why cant my 5 yr old choc lab manage an hours walk?

Question
hi there, my 5 yr old choc lab bitch is really struggling with long walks at the moment, ive recently started using a harness as when i used the lead she would pull me towards other dogs and strangle herself and collapse which was really worrying, so today i was quite hot but not boiling, and i walked her at 9am so not really hot!...she was ok for 3/4 of the walk, then whenever she saw shade she would just lie down and try and cool down, i took water with me and keep giving her water, and poured it over her, then for the rest of the walk she just walked really really slowly!!....is she just lazy or would it be a medical issue??...many thanks

Answer
I think I just answered this on another site.  

Water, water, water. I take my Labs out a lot in hot weather. I do set a slow pace and take breaks in the shade. They get plenty of water to drink and ice cubes. I also keep them wet. Does your route include a pond or something for a dip?

We crazy Easterners once went walking in the Nebraska Sand Hills in the summer heat with my sister's dogs. They had a great time, hitting the irrigation canal as needed. They took the heat better than we did.